Effects of radiation on fine vasculature may determine the response of dependent tissues late after a course of radiotherapy. Additional stress to irradiated tissues may cause injury to fine vasculature leading to degeneration and necrosis. It is difficult to study radiation dose response and repair capability of nonproliferative or slowly proliferating mammalian tissues. However, knowledge about radiation repair by normal tissues becomes critical when considering use of radiotherapy techniques which may alter this capability. The primary objective of this research is to determine single dose and split-dose radiation survival curves for endothelial cells. Injury to the cornea of the eye will cause proliferation of capillary endothelial cells at the limbus. Irradiation prior to and following induction of capillary proliferation permits evaluation of response of both slowly and rapidly proliferating capillary endothelial cells. Morphometric analyses of capillary volume has permitted dose-response assays. The "Do" values obtained were 376 rads for 60Co gamma radiation, 314 rads for neutrons and 214 rads for negative pi mesons. RBE's determined by comparing doses required to reduce capillary volume to control values were 1.96 for neutrons and 1.66 for pions. Experiments will be continued with both low LET and high LET and thermal irradiation, as this may ultimately have practical radiotherapeutic application. Studies will also be done with combined ionizing radiation and chemotherapeutic agents. Radiation response of heart and kidneys have been instituted. Correlations are being made between early changes in serum isoenzyme patterns and late histologic damage to the organs.